THRIFT-495. python: Raise a TTransportException if TSocket is not open

THRIFT-495. python: Raise a TTransportException if TSocket is not open

Previously, TSocket.write would raise an AttributeError if the transport
had not been opened.  A TTransportException with the code set to
NOT_OPEN is more appropriate.  The cost of the check should be
negligible, since TSocket.write is already fairly expensive (it does a
system call).

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@773762 13f79535-47bb-0310-9956-ffa450edef68